The Commission shall by regulation establish a classification of goods and services for convenience of the administration of this chapter, but not to limit or extend the applicant’s or registrant’s rights, and a single application for registration of a mark may include any or all goods upon which, or services in connection with which, the mark is actually being used indicating the appropriate class or classes of goods or services. When a single application includes goods or services which fall within multiple classes, the Commission may require payment of a fee for each class. To the extent practical, the classification of goods and services should conform to the classification adopted by the United States Patent and Trademark Office.